1. Field of the Invention
The present invention relates to leather-like materials having a soft feel and an excellent water vapor permeability and comprising a grain layer, particularly, a thin, microporous grain layer, having an animal skin-like grain appearance. This invention also relates to processes for manufacturing these leather-like materials by utilizing the so-called "wet regeneration process".
2. Description of the Prior Art
As a leather-like material, those of light weight having a soft feel have so far been desired. Therefore, leather-like materials of light weight which have as thin a grain layer as possible have been most extensively used. Such leather-like materials are manufactured generally according to the so-called "dry, release paper process" wherein a grain patterned release paper is coated with a resinous material solution for a grain layer and then the resulting resinous film imparted with a grain appearance is transferred onto a leather substrate, followed by drying. However, this process produces nonporous grain layers and, therefore, it can not help from sacrificing desirable functions, such as water vapor permeability.
On the other hand, there has been widely used the so-called "wet process" wherein a substrate comprising a fiber aggregate, such as woven fabrics, nonwoven fabrics, knitted goods or the like, which has been pre-finished with a binder, is coated with a resinous material solution for a grain layer and then a grain layer is formed by means of wet regeneration. According to this process, though microporous grain layers can be obtained, the thickness of the grain layers must be increased (in most cases, up to 300.about.500 .mu.m) so that the grain layers can cover up recesses and projections on the substrate, such as the texture of woven or knitted fabrics, fluffs of nonwoven fabrics, binders or the like. Therefore, the feel of the products inevitably becomes hard and stiff. Moreover, re-dissolution and re-coagulation of the binder in the substrate, due to the coated grain layer resinous material solution, extremely enhance hardening and stiffening, so that its use has had to be limited.
In order to produce leather-like materials having excellent suppleness, water vapor permeability and surface appearance by providing a substrate with a very thin and flat microporous grain layer, particularly 300 .mu.m thick or less, preferably 200 .mu.m or less, many attempts to improve the coating means, such as doctor coating, reverse roll coating, gravure coating, spray coating or the like, heretofore have been made. However, even though grain layers more than about 300 .mu.m thick could be produced by any means, such an extremely thin layer as desired by the present invention has been almost impossible to manufacture by only improving these coating means. One of the primary technical reasons for difficulties encountered in the manufacture of a smooth and flat surface, thin microporous grain layer according to the conventional processes is that resinous materials for the grain layers contain a solvent in an amount of not less than 70.about.85% by weight which diffuses into water to greatly reduce the thickness when the grain layer resinous materials coagulate to set up, so that the recesses and projections on the surface of the substrate are emphasized to produce a rough surface. As a process aiming to eliminate these technical drawbacks, there have been proposed water vapor coagulation processes as disclosed in, for example, Japanese patent application Publication Nos. 4,434/1962 and 48,506/1972. These processes are techniques to obtain porous grain layers by contacting the surface of the coated grain layer solution directly with water vapor to coagulate the solution while being maintained as a flat, horizontal plane. According to these processes, since a relatively large quantity of grain layer resinous material solution is coated, smoothness can be readily obtained by virtue of the principle of the horizontal liquid plane. However, the thickness of the resulting grain layer is generally from more than 300 to 500 .mu.m and, as described hereinbefore, stabilized, thin grain layers 300 .mu.m thick or less, preferably 200 .mu.m thick or less, have not been able to be obtained. Besides, the grain layers according to these processes have pores of a relatively large diameter formed in the vicinity of the surface, so that the surface structure materially collapses during the embossing step, causing a problem in abrasion resistance.
As described above, leather-like materials which are excellent in both water vapor permeability and suppleness, and comprise a thin, microporous grain layer having a smooth appearance have been strongly desired in the market, whereas such products have not appeared as yet.
Alternatively, leather-like materials having a grain layer of a smooth surface, in most cases, are further subjected to a conventional embossing process to increase their commercial value, whereby it cannot be denied that the grain layer of the microporous structure has collapsed due to the heat and pressure applied during embossing so that the suppleness thereof is impaired.
Accordingly, leather-like materials which have excellent water vapor permeability and suppleness, and comprise a thin, microporous grain layer with a desired clear animal skin-like grain appearance, without impairing these excellent properties, have also been desired to be brought into the market.